Infrared rays are radiated from a heat source even in the dark, and characteristically have higher permeability than visible light in smoke or fog. Accordingly, infrared imaging can be performed during day and night. Also, temperature information about an object can be obtained through infrared imaging. In view of this, infrared imaging can be applied to a wide variety of fields such as the defense field and the fields of security cameras and fire detecting cameras.
In recent years, “uncooled infrared imaging devices” that do not require cooling mechanisms have been actively developed. An uncooled or thermal infrared imaging device converts an incident infrared ray of approximately 10 μm in wavelength into heat with an absorption mechanism, and further converts the temperature change at the heat sensitive unit caused by the small amount of heat into an electrical signal with a thermoelectric conversion means. The uncooled infrared imaging device reads the electrical signal, to obtain infrared image information.
For example, there have been infrared sensors each using a silicon pn junction that converts a temperature change into a voltage change by applying a constant forward current. Characteristically, such infrared sensors can be mass-produced through a silicon LSI manufacturing process using a SOI (Silicon On Insulator) substrate as a semiconductor substrate. Also, the rectification properties of the silicon pn junctions serving as the thermoelectric conversion means are utilized to realize the row select function, and accordingly, the pixels can be designed to have very simple structures.
One of the indicators of the performance of an infrared sensor is NETD (Noise Equivalent Temperature Difference), which indicates the temperature resolution of the infrared sensor. It is critical to reduce the NETD, or reduce the infrared sensor temperature difference equivalent to noise. To do so, it is necessary to increase signal sensitivity and reduce noise.
Thermoelectric conversion elements are sensitive to temperature components other than temperature rises caused by incident infrared rays, or to the temperature of the semiconductor substrate and the self-heating temperature at the time of flowing of current. To correct those “offset temperatures”, reference pixels are provided.
Like an infrared detection pixel, a conventional reference pixel reflects the influence of the temperature of the semiconductor substrate, but has a different self-heating temperature from that of an infrared detection pixel. The difference in self-heating temperature is much larger than a signal generated from an incident infrared ray, and therefore, it is necessary to correct the difference.